Simplify the following expression: $y = \dfrac{7}{8q} - \dfrac{7}{9q}$
Answer: In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $8q$ and $9q$ $\lcm(8q, 9q) = 72q$ $ y = \dfrac{9}{9} \cdot \dfrac{7}{8q} - \dfrac{8}{8} \cdot \dfrac{7}{9q} $ $y = \dfrac{63}{72q} - \dfrac{56}{72q}$ $y = \dfrac{63 -56}{72q}$ $y = \dfrac{7}{72q}$